Geological Society. 239 



form, when it is clear that they are not fused together into a con- 

 tinuous network, ; they form a network only by the interlacing of 

 their extremities. The spicules are quadriradiate, with the centre 

 raised, so that each spicule indicates the outlines of a low four- 

 sided pyramid, the centre being at the apex, and the four rays re- 

 presenting the four edges of the pyramid. The rays do not diverge 

 at right angles ; and thus the base of the pyramid is oblong, though 

 this may be due to distortion. From some indications the author 

 is inclined to believe that a fifth ray may have sprung from the 

 centre of the spicule downwards. The rays of the spicules appear 

 to be cylindrical. The spicules are generally of several sizes, the 

 larger ones forming a framework which is filled in by the smaller 

 forms, the latter being regularly arranged ; so that the smaller ones 

 fill up the square spaces left between the rays of the larger, and 

 thus build up a network of square meshes gradually diminishing in 

 size. The sponge-wall seems to have consisted of more than one 

 layer of spicules. The sp'cules were probably originally siliceous ; 

 but now they consist of iron pyrites. 



With regard to the systematic position of Protospongia, the oldest 

 known sponge, the author remarks that similar spicules similarly 

 arranged are to be met with in the Hexactinellidae, the absence of 

 one or two rays being not unusual in part of the spicules of true 

 Hexactinellids. As the spicules are free, he would refer the sponge 

 to Zittel's Lyssakina, which are nearly equivalent to Carter's Sarco- 

 hexactinellida. 



2. " Note on Psephophorus polygonus, von Meyer, a new Type of 

 Chelonian Reptile allied to the Leathery Turtles." By Prof. H. G. 

 Seeley, F.R.S., F.G.S. 



The specimen described is a portion of the shield from the an- 

 terior dorsal region, and was obtained from the later Tertiary 

 deposits of the borders of Croatia. It was originally regarded by 

 Von Meyer as the armour of an Edentate mammal ; but it was 

 afterwards suggested by him and Prof. Fuchs that Sphargis pre- 

 sented a nearer affinity. A keel runs along the middle of the 

 specimen, and is regarded by the author as one of the outer folds 

 of the shield. The dermal skeleton is made up of irregularly poly- 

 gonal plates of various sizes, closely resembling those of Sphargis, 

 except that each plate is almost twice as large as those of that 

 form. The plates usually show a radiate ornament on the surface. 

 On the underside of the slab are the remains of several vertebrae, 

 apparently from the base of the neck ; and these differ from the 

 vertebrae of all known Chelonians in having strong transverse 

 processes for the attachment of ribs. The neural arch, like the 

 processes, is ankylosed to the centrum. The author considers 

 that the dermal skeleton is not represented in the carapace of ordi- 

 nary Chelonia, but is represented by the granulations on the surface 

 of the carapace of the Trionychidae. He is hence led to indicate 

 three primary divisions of the Chelonian order, viz. : — 1. Aspidoche- 



17* 



